The present invention relates to a method for closed-loop control of injection timing in combustion engines, based upon analysis of the characteristics of the ionisation current as detected via a measuring gap arranged within the combustion chamber.
Lambda sensors are often used in order to obtain closed loop control of stochiometric combustion in combustion engines. A stochiometric combustion is the ideal operation mode for a conventional three-way catalytic converter. The type of lambda sensors used in mass-produced cars have been a so-called narrow-banded lambda sensor, which type of sensors exhibit a distinct transition as of its output signal at a lambda value just below 1.0. This type of narrow-banded lambda sensor is used in order to control the combustion, wherein the control is operated such that the output signal of the lambda sensor switches between a low or high output signal.
An alternative to the narrow-banded lambda sensors is the linear type of lambda sensors, but these sensors are very expensive, at least 10-fold, and could therefore in terms of cost not justify an introduction in mass-produced cars. The linear type of lambda sensors emits an output signal proportional to the present air/fuel ratio. By using lambda sensor of the kind mentioned above, the amount of fuel being supplied could be controlled, in order to maintain a defined air/fuel ratio, i.e. A/F-ratio (Air/Fuel).
An alternative to lambda sensors is shown in U.S. Pat. No. 4,535,740 having an ion current sensor in the combustion chamber where the spark gap of the conventional spark plug is used as measuring gap enabling detection of the burn duration within the combustion chamber. A parameter representative of the burn duration, and thus the air/fuel ratio, is detected by measuring the length of time the ion current signal is above a predetermined threshold value. At certain operating ranges where the ion current signal exhibit a low accuracy, the closed loop control is based upon the termination of the burn duration. The characteristics of the burn duration vary considerably at different operating cases, i.e. load and rpm's, and for that reason alone there is a need for a number of different threshold values to be used for the detection of burn duration, or alternatively of using different weight factors for different load cases.
In U.S. Pat. No. 5,425,339 another closed-loop system is shown, wherein information from the ionisation current is used in order to control ignition timing and the amount of fuel being supplied, i.e. the present A/F ratio. In this implementation the product of the duration of the ionisation signal and the peak value is maximised, during variation of the fuel amount being supplied or alternatively during variation of the ignition timing. By duration of the ionisation signal is meant the time the ionisation current exceeds a predefined threshold level. In an alternative embodiment the integrated value of the ionisation current signal could be maximised during variation of either the fuel amount or the ignition timing.
In U.S. Pat. No. 5,425,339 is also shown (FIG. 5) a previously known system configuration, wherein also the injection timing could be controlled in a conventional manner. This conventional manner uses a feed-forward model, using empirically determined matrices for the injection timing, wherein the dominating parameters such as rpm, load and temperature all determine the present injection timing. Nothing in U.S. Pat. No. 5,425,339 suggests that the injection timing should be controlled in a closed-loop manner dependent on the ionisation current signal.
In SE,C, 503900 is shown an alternative for detecting the present A/F-ratio. Instead of using the duration of the ionisation current signal in order to determine the A/F ratio, as shown in U.S. Pat. No. 4,535,740, it is suggested in SE,C,503900 that a parameter extracted from the ionisation current signal and typical for a basic frequency content instead be used for this A/F detection. As an alternative the derivative of the ionisation current signal during the flame ionisation phase could be used, which derivative value is directly dependent an the basic frequency content of the ionisation current signal.
Further stringent demands on reduced emission levels have resulted in combustion engines being operated with ultra lean air-fuel mixtures, as seen globally in the entire combustion chamber, and wherein a stratification of the air-fuel mixture is needed with a richer mixture locally around the ignition plug. With combustion engines having direct injection this stratification could be obtained, by injecting the fuel amount in the vicinity of the spark plug shortly before initiating the ignition. By this stratification technique, so called stratified charge, combustion could be initiated properly with an ideal A/F ratio in the spark plug gap for the present operating conditions. Even though extensive testing and evaluation is performed in order to determine when the fuel is to be injected into the operating cycle of the engine, a problem will occur because different engines behave differently in aspects of how the fuel mixture moves inside the combustion chamber during different rpm's and loads. This could result in an ideal A/F-ratio not being obtained in the spark plug gap when ignition is to be initiated.
For example, in certain engines irregularities in the inlet manifold, caused by residual burrs from casting of the manifold, could have an impact upon the air flow into the combustion chamber such that the developed horizontal rotation (i.e. swirl), or the vertical rotation (i.e. thumble), is not developed in the same manner between cylinders.
During regular production also different engines could obtain different air flow into the combustion chambers, in comparison to the reference engine or engines being used in order to determine when fuel is to be injected into the combustion chamber. Different operating conditions with regulated amount of recirculated exhaust gases, so called EGR-control, could also have an uncontrolled impact upon the air motion between successive combustion's and also cause a gradual change in the order of impact in the long run.
If the timing of fuel injection into the combustion chamber is determined in such a manner that the fuel cloud deposited after finished injection should move a certain distance within the combustion chamber, by assistance of the developed air motion within the combustion chamber and before the fuel cloud reaches the ignition plug and ignition is initiated, then it is of extreme importance that the air motion is controlled. If the air motion is subjected to large variations between different cylinders but also between different operating conditions, then also the injection timing must be adapted to the existing air motion of the cylinder and present operating condition.